The same session run on both instances (RAC), is it normal?

We have a two nodes RAC, the strange thing is that many sessions run on both
nodes.
      select sid from gv$session group by sid having count(*)>1
they are lots of rows return.

My understanding for load balance is:  if sessions on instance 1 are too
many, then new session will be connected to instance 2.
But if at the beginning there were100 sessions on both inst1 and inst2,
after all the sessions on inst2 are completed, no existing session on
instance1 will go to instance2.

But the fact of the above query told me some sessions run on both instance.
How can this be true?

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